It is assumed that these compound dyes act on the protoplasm 

 somewhat as follows : Certain parts of the cell have an affinity for 

 the neutral stain and take it up as such; others, having an affinity 

 for the basic dye, break up the neutral stain so as to obtain the 

 basic portion of it, or if dissociation has taken place, take up the 

 basic ion directly; while other parts of the cell with an affinity for 

 acid dyes similarly combine with the acid portion of the stain. 

 These three types of cell structures are known as neutrophile, 

 basophile and oxyphile elements, respectively. The differentia- 

 tion thus produced gives the neutral dyes their great value. 



Ehrlich's"Triacid Stain." 



The first neutral stain proposed for microscopic work was the 

 "triacid stain" (seeEhrlich 1910, 1, 227). In forming this compound 

 dye, acid fuchsin and orange G are mixed in solution and to the 

 mixture is then added such a quantity of methyl green that there 

 is still an excess of the acid dve. This excess of the acid dve allows 

 the neutral stain to stav in solution. The dve thus formed is a 

 very valuable blood stain, and brings out finely the different struc- 

 tures in the leucocvtes. 



Slight modifications of this triacid stain have been used for 

 tissues. The best known of these modifications is that of Biondi- 

 Heidenhain. 



Eosin-Methylene-blue Compounds* 



The first worker to combine eosin and methylene blue was 

 Romano vsky (1891). He realized that a mixture of these two dyes 

 had great selective properties as a stain, and showed it to be ex- 

 cellent for blood, particularly in bringing out the malarial parasite. 

 He also appreciated that it was more than a mere mixture of the 

 two dyes and that some new dye having the property of giving the 

 nuclei a red color was present. It was some time later before the 

 nature of this new dye was known, alt ho it was subsequently 

 named azure I or methylene azure; its true chemistry has scarcely 

 been understood until very recently (see p. 48). Methylene violet, 

 which probably was also present, had already been described by 

 Bernthsen (1885) . How these new dyes were formed in the Roman- 

 ovsky stain was not known then; altho Romanovsky stated that 

 different lots of methylene blue solution varied in their ability to 

 give a good blood stain, and that old solutions on which a scum 

 had formed were best. 



Present day blood stains are often spoken of as modified Roman- 

 ovsky stains; altho the modifications are so great as to make them 

 of a very different nature. The first modification was made by 

 Nocht (1898) who concluded that the differential staining was due 



*A good account of the history of these blood stains is given by MacNeal (1906). 



88 



